The present invention relates to cooling systems in hybrid vehicles of the so-called "electric-hybrid" (EH) type, namely of the type having a main electric drive and an auxiliary internal combustion engine (AICE) drive.
Environmental issues have heightened the interest in recent years in alternative means for providing personal and commercial transportation. Economic and regulatory issues have combined to promote the view that electric powered vehicles will, over the next ten years, appear in significant numbers. It is possible that the number of electric or electric-hybrid (EH) vehicles in key areas may be around 100,000 or more by the year 2000.
Aside from the known modest performance levels of electric and hybrid vehicles, a major issue is that of cost. Presently, EH vehicles with performance levels acceptable to personal users are expected to sell from 2 to 2.5 times the price of functionally comparable conventional vehicles. This cost differential is in part due to the cost of the batteries needed, but is also due to the many support systems needed in an electric vehicle. The support systems problem is even more severe in an EH vehicle.
Such support systems include vehicle heating. Conventional vehicles provide interior heating by utilising waste heat from the internal combustion engine cooling system. This is not available in electric vehicles, or in hybrid low emission vehicles which operate for much of the time in electric-only mode. Electric vehicles typically provide the 1 to 2kW of heat required directly from the traction battery. Consequently, one hour of heating may absorb 1kW-hr or more. This energy level is significant in comparison with the battery energy storage capability of 15 to 25kW-hr typical for personal electric and EH vehicles.
Aside from the subject of vehicle support systems, a specific problem arises with EH vehicles which use auxiliary internal combustion engines. To minimise exhaust emissions, the operating strategy of these vehicles is structured to run the internal combustion engine as little as possible, and to operate the internal combustion engine at points of best emissions, when it is running. These best emission points are typically at close to the maximum engine output. Consequently, the internal combustion engine is started and stopped frequently.
Following start, a period of typically two minutes or longer will be required before the internal combustion engine reaches operating temperature and can reliably accept high loads. During this "warm-up" period the engine emissions are high. Indeed, emissions during the warm up period account for a significant proportion of all vehicle emissions.
Conventional EH vehicles use a so-called "series hybrid" approach in which the auxiliary internal combustion engine is connected to a dedicated generator and is not used directly to drive the vehicle. An alternative approach, the "parallel hybrid" has both parts of the drive mechanically connected to the driving wheels. This has some advantages, particularly the ability to supply considerably more power than the electric drive alone and the elimination of the separate, expensive generator. Where the vehicle motional power demands are below the maximum output of the AICE, the AICE is run at a high output level (preferably a point of minimum emissions) and the electric traction drive is operated as a brake. The excess AICE output, over the vehicle motional demands, is then regenerated to the batteries via the traction drive.
However, a weakness of such "parallel" systems is the vehicle performance over extended periods of slow speed operation; for example, crawling in congested traffic conditions. Clearly, this mode of operation is important in a vehicle intended for use in major conurbations, such as Los Angeles. Under these conditions the AICE cannot be used to charge the batteries as the driveline speed is for the most part below the minimum running speed of the AICE, and the AICE must be disengaged. Complete depletion of the batteries then becomes possible, regardless of the amount of fuel available for the AICE.
The traction system of an EH vehicle may utilize well-known pulse-width modulation (PWM) inverter methods to synthesize a closely-controlled AC supply, from the DC traction battery. The controlled AC supply is used to drive conventional induction, permanent magnet synchronous or other motors under variable speed and torque regimes to meet the demands of the vehicle user. A major advantage is the brushless nature of the motor, which has markedly lower cost--and higher environmental tolerance--than the brushed DC motors normally used for controllable drives.
Since 1985, drive systems of the AC type have come into more general use. An industrial inverter drive normally operates by first converting the normal three phase or single phase line supply to an intermediate DC voltage, prior to "inverting" the DC back to AC with the desired parameters for driving the target motor. This intermediate rectification process complicates the drive, adding to cost, and has played a part in slowing the spread of AC inverter drives for industrial applications. An AC drive is, however, well suited to vehicle traction applications where the primary energy source is DC batteries. Vehicle applications of AC systems are still in the minority compared to conventional DC brushed traction systems, primarily due to the sophistication of the control systems necessary to achieve satisfactory operation with the AC system--and the costs of such systems when conventional methods are used.
The inverter proposed for use in the present system is based on insulated-gate bipolar transistors, operating under the control of a microcomputer. Many other device technologies are also applicable, and other control methods aside from PWM can also be used. An example is the Load Commutated Inversion method, relying on natural commutation of the inverter devices, which is particularly applicable with permanent magnet machines.
Such an inverter has an energy efficiency of approximately 96% at full load, so that when 50kW are being delivered to the traction motor or motors, 2kW is dissipated in the inverter. At lower power levels, the losses are not as substantial. However, it is rare for the losses to be lower than 1kW. It is normal in vehicle drive systems to use air cooled thermal radiators or heatsinks to directly dissipate this heat to the ambient air.